Crystals of monocrystalline silicon which is essential for production of semiconductor devices, etc. are grown by a method such as the CZ method or the FZ method, and a polycrystalline silicon rod or a polycrystalline silicon ingot is used as the raw material. Such a polycrystalline silicon material is in most cases produced by the Siemens method (see Patent Literature 1). The Siemens method is a method involving bringing a source gas of silane such as trichlorosilane or mono-silane into contact with a heated silicon core wire and thereby inducing vapor-phase growth (deposition) of polycrystalline silicon on the surface of the silicon core wire by the chemical vapor deposition (CVD) method.
For example, when crystals of monocrystalline silicon are grown by the CZ method, a polycrystalline silicon ingot is charged into a quartz crucible, and a seed crystal is immersed in a silicon melt, into which the polycrystalline silicon ingot is heated and melted, to delete dislocation lines and obtain a dislocation-free crystal, and thereafter the crystal is gradually increased in diameter until a predetermined diameter is reached and then pulled up. During this process, if unmelted polycrystalline silicon remains in the silicon melt, this unmelted polycrystalline piece floats around the solid-liquid interface due to convection and causes loss of crystal lines by inducing generation of dislocation.
Patent Literature 2 reports that needle crystals may be deposited during a process of producing a polycrystalline silicon rod by the Siemens method, and that if single-crystal silicon is grown by the FZ method using such a polycrystalline silicon rod, due to the above-mentioned inhomogeneous microstructure, individual crystallites are not melted homogeneously according to their sizes, but the unmeltable crystallites pass through the melting zone as solid particles toward the single-crystal rod and become incorporated into the solidification surface of the monocrystal as unmelted particles, thereby causing defect formation.
To address this problem, Patent Literature 2 proposes a technique, in which the surface of a sample, which is cut out perpendicularly to the longitudinal direction of the polycrystalline silicon rod, is ground or polished, and after etching, the contrast of the microcrystals of the structure is enhanced to such a degree that the microcrystals are visible even under an optical microscope, and the size and area ratio of needle crystals are measured, and on the basis of the measurement result, whether or not the polycrystalline silicon rod is acceptable as a raw material for growing single-crystal silicon by the FZ method is determined.